1. Field of the Invention
The invention concerns a method for inspecting a weld seam. The weld seam between the two materials is produced by a laser beam One of the plastic materials of which the workpiece is made is essentially transparent to this laser radiation, whereas the other plastic material absorbs the laser radiation.
2. Description of the Related Art
In the previously known method of this type, the inspection was performed with a pyrometer, which responds to the thermal radiation emitted by the weld seam that has been produced. For the pyrometer to reach the highest possible temperature, the measurement must be made during welding. In addition, the material of which one of the plastic parts is made must be capable of transmitting thermal radiation.
Laser radiation has proven to be very effective for welding plastics. There are three welding methods for laser radiation, namely, “track welding”, “simultaneous welding”, and “quasi-simultaneous welding”. Pyrometric inspection of the weld seam is possible only with track welding and cannot be used in simultaneous welding or quasi-simultaneous welding, although the latter provides a time advantage over track welding. In the case of a welded product produced by the simultaneous welding method, inspection of the welded materials could be made only indirectly, by dimensional changes in the workpiece and could only be used with certain workpiece geometries. The quality of a simultaneous weld or quasi-simultaneous weld generally could not be inspected.
The pyrometric weld inspection possible in the case of track welding is also subject to error and can be used only if the material of one of the plastic parts transmits not only laser radiation but also thermal radiation. Therefore, in many cases, weld inspection of the finished workpiece is not possible at all.
In one well-known method of a different type (DE 196 03 675 A1), the weld joint is produced by contact welding of two superimposed plastic sheets from which a bag is to be produced. Each of the two plastic sheets itself consists of two layers, namely, a transparent, infusible outer support layer and a colored inner sealing layer. The contacting sealing layers of the two sheets are welded together by two heated sealing jaws pressing against each other. As a result of the weld joint on the colored sealing layers, the transparency of the weld seams changes relative to places that were not welded or were not adequately welded. These transparency differences are determined by a light transmission method and used to inspect the quality of the weld joints. The two sheets are transilluminated in the region of the weld seam by a light source, and the light emerging at the other side of the sheet is detected by a sensor and analyzed. This well-known method cannot be applied to weld seams produced by laser radiation, because one of the plastic parts of this welded product is absorbent and therefore opaque.
In another method for inspecting weld seams in bags produced from two sheets, which are then to be immediately filled with some substance (U.S. Pat. No. 5,260,766 A), laser light is projected into a transparent heated seal bar through a large number of glass fibers. In this way, the light reaches the contact area between this heated seal bar and an opposing bar, between which the two sheets are positioned and sealed. The light reflected from this contact area must pass back through the transparent material of the heated seal bar to produce an image, which is picked up by a camera and analyzed to determine whether particles of the material used to fill the bag are enclosed in the weld seam. This makes it possible to determine the quality of the weld seam. This method can be used only with thin flat sheets in which linear weld seams are formed and requires a transparent welding tool. It cannot be applied to the laser welding of three-dimensional plastic parts with two-dimensional or three-dimensional weld seams, especially if two plastics with different optical and/or mechanical properties are to be welded together.
Furthermore, it is also well known (DE 298 16 401 U1) that a transillumination technique can be used to detect cracks in a welded lap joint produced by the lap welding of sheets. In this method, the weld seam is placed between an optical transmitter and an optical receiver. To increase the accuracy of the measurement, this transillumination technique is carried out in a liquid with an extremely low viscosity. This method is not suitable for the inspection of weld seams produced by laser radiation between two plastic parts, one of which is absorbent.
Finally, it is well known (JP 10[1998]-100,259 A, Patent Abstracts of Japan, Vol. 1998, No. 09, Jul. 31, 1998) that two similar polyethylene materials can be irradiated with broadband infrared radiation. As long as the resulting weld seam is in the liquid state, the infrared radiation reflected by the liquid or passing through the liquid is optically detected and analyzed. A laser beam is not used. Inspection radiation is not used in addition to the infrared radiation, so it does not matter whether one of the polyethylene materials is transparent to inspection radiation.